Automatic phototimers of the type having a fluorescent screen positioned behind or beneath a subject in the path of an x-ray beam together with one or more phototubes which receive the light from the screen are well known. An example of one such phototimer is described in U.S. Pat. No. 3,752,991 to Slagle, owned by the present assignee and which is hereby incorporated by reference.
The purpose of a phototimer is to terminate an x-ray beam when a film cassette, positioned in the beam, has received an exposure that will produce an image on the film of desired density in the zone of the "active" sensing area.
To control the timing of the exposure, a phototube receives light generated by incident radiation on a scintillator screen. The phototube generates a signal proportional to the radiation dose incident on the screen. The signal is integrated over time and compared to a reference signal. When the integrated signal reaches a level equal to the reference signal, a termination signal effective to terminate the exposure is produced.
Traditionally, the reference signal was produced using a simple kVp density compensation scheme, i.e., the reference signal varied as a linear function of selected kVp. For a low range of selectable kVp, e.g., 50-75 kVp, the value of the reference signal declined linearly as selected kVp increased. Over a second range, e.g., 76-150 kVp, the value of the reference signal was a predetermined constant value. Density variation in the resultant image was achieved by shifting the reference signal versus kVp curves up or down by a factor corresponding to the desired density variation.
A simple linear relationship between selected kVp and the reference signal has proved inadequate to properly compensate for variation in film density over the range of selectable kVp. Additionally, breaking the compensation scheme down to only two regions has not provided the desired degree of control. Also other factors besides selected kVp and desired variation in film density need to be considered in the overall compensation scheme.
In another compensation scheme described in U.S. Pat. No. 4,454,606 compensation curves representative of plots of reference signal v. exposure time can be reconfigured to account for the variable effects on x-ray film density resulting from a variety of system parameters. This scheme is useful in providing compensation for short (e.g., &lt;20 ms) exposure intervals but is inadequate for longer exposure and doesn't adequately account for kVp density variations.
It has also been found desirable to provide a control circuit which can selectively integrate a plurality of PMT signals, compensate these signals for stray leakage currents such as PMT dark current, mix the selected signals, selectively amplify and gain factor the mixed signal to account for the number of phototimer fields selected and the speed of the film and intensifier screen chosen.
In the above referenced Slagle patent, a multi-field, single PMT system is described. Movable shielding means, which formed a part of the photomultiplier tube housing structure, was utilized to selectively block the transmission of light from the field to the single PMT. The use of a single phototube results in less than optimum light collection efficiency from the sensing areas since only a portion of the light sensitive area of the phototube is energized. Further, the movable shielding means comprise moving parts that are prone to wear resulting in poor long term performance and reliability.
In other phototimer designs, automatic compensation of a number of error causing signals associated with a photomultiplier tube, e.g., dark current, was provided. An example of one such compensation scheme can be found in U.S. Pat. No. 3,600,584 which describes dark current compensation of a single field, single tube device.
Other single detector devices provide means for varying the gain of the integrated output to account for different film speeds commonly used. An example of one such device is U.S. Pat. No. 4,250,103.
In yet other designs, multiple fields are provided. Means for selecting any one of the fields or any combination of the fields is provided. An example of one such device can be found in U.S. Pat. No. 4,230,944.
None of these designs offer the advantage of combining all of the features into a single control circuit in order to achieve the reliability, flexibility and control over the exposure offered by the present invention.